Results and Discussions To verify the validity of the simulation analysis, we build an experimental setup (Fig. 6) and conduct experiments on the capture and capture stability of Chlorella cells by using optical tweezers. The 980 nm SMF is connected to a laser source with a wavelength of 980 nm as the input, and the 1550 nm SMF is immersed in the Chlorella cell solution as the output. When the laser is turned on, the Chlorella cells in the effective trap area of the optical fiber probe are attracted to the fiber tip. Due to the stronger light field gradient distribution on both sides of the fiber tip, the Chlorella cells are trapped on both sides of the fiber tip (Fig. 7). The experimental results show that the optical tweezer structure is able to simultaneously trap multiple Chlorella cells in three directions and form a biological chain. The capture remains stable when the fiber moves at a speed of about 14 μm/s (Fig. 9).

Currently, some biophotonic devices or cell-to-cell interactions and communications require the capture of particles, especially multiple particles. Since the invention of optical tweezers in 1986, optical tweezers have become an important tool that is widely used in the manipulation and study of cells, viruses, atoms, colloids, and other particles. Based on conventional optical tweezers, multi-directional alignment of multiple particles is achieved by various methods such as holographic optical tweezers, single beam before helical phase, and optical binding. However, these techniques require bulky optical components, which complicates optical tweezer systems and hinders operational flexibility. To overcome the shortcomings of conventional optical tweezers in capturing multiple particles, researchers have used optical tweezers to capture multiple particles. Some researchers have created multiple optical traps using dual fibers, which enables the capture of multiple particles in two and three dimensions, and they have manipulated, deflected, and stretched multiple cells using two misaligned single-mode fibers. Some researchers have used multicore fibers for two-dimensional optical interference capture of multiple particles and Escherichila coli cells manipulation of multiple particles using photonic crystal mode multiplexing, while others have used fiber traps and photothermal effects to manipulate a large number of particles. However, the optical fiber probes in the above methods with multi-core fibers and photonic crystals are, in general, structurally complex, and difficult to replicate. Focusing on the complex structure of multi-core fiber and photonic crystal fiber probe, this paper proposes a single-fiber optical tweezer structure with two modes being composite. The structure utilizes two different modes of fiber staggered splicing to ensure the LP₀₁ and LP₁₁ modes coexist in the output optical field, and the two modes of the beam have different focused optical fields to achieve the capture of multiple Chlorella cells in different directions. The captured Chlorella cells act as lenses to refocus the beam to capture the next cell and then form multiple biological chains.

In order to make LP₀₁ and LP₁₁ mode beams coexist in the fiber, 980 nm single mode fiber (SMF) and 1550 nm SMF are utilized for splicing (Fig. 1). The energy ratio of the LP₀₁ and LP₁₁ mode beams is also controlled by controlling the offset of the two fiber splices, which in turn ensures that each optical trap can have sufficient optical power to trap particles. In order to analyze the focused optical field characteristics of the composite mode fiber, a two-dimensional model based on finite element analysis is developed using simulation software. The output optical field distribution of the composite fiber with 980 nm SMF and 1550 nm SMF staggered by 2 μm is simulated, and the optical radiation pressure applied to Chlorella cells is calculated. The simulation results show that the LP₀₁ mode beam is focused at the tip of the fiber probe and forms an optical potential well [Fig. 4 (a)]. The LP₁₁ mode has a completely different light field at the tip of the fiber probe [Fig. 4 (b)]. The LP₁₁ mode light field is not concentrated near the optical axis. The convergence position of the LP₁₁ mode beam is inside the fiber tip. Due to the special fiber shape, the light field gradient distribution on the side of the fiber is large, so Chlorella cells outside the fiber tip will be attracted and move toward the fiber tip and eventually be captured. The coexistence of the LP₀₁ and LP₁₁ mode beams integrates the characteristics of both LP₀₁ and LP₁₁ mode beams [Fig. 4 (c)]. The LP₀₁ mode beam is also present while the LP₁₁ mode beam is excited in the fiber, and the two-mode beams exhibit different focused light fields because they have different propagation constants. In other words, the LP₀₁ and LP₁₁ modes produce different stable capture points when passing through the same fiber probe. When LP₀₁ and LP₁₁ modes coexist, the simulation results show that Chlorella cells are captured on both sides of the optical axis and the fiber tip, respectively (Fig. 5).

In summary, a single-fiber optical tweezer for multiplexed alignment of multi-biological cells is proposed in this paper. The optical tweezer utilizes two different modes of fiber staggered splicing to make LP₀₁ and LP₁₁ modes coexist in the output optical field. Since the two mode beams have different propagation constants and exhibit different focused light fields, the capture of multi-biological cells in different directions can be achieved. Through the finite element analysis method, the optical field distribution of the optical fiber tweezer with 980 nm SMF and 1550 nm SMF being composite is simulated, and the force on Chlorella cells is analyzed. Finally, it is shown that the optical tweezer can capture multiple Chlorella cells simultaneously in three directions and form a biological chain. The capture remains stable when the fiber travels at a speed of about 14 μm/s. The simple structure of this optical fiber tweezer provides more possibilities for biosensing and direct detection of biosignals.